Download KEY - Practice Problems for Genetcis Test

KEY: The formation of gametes
1. Kelp, a large, multicellular type of algae, has air bladders in the blades to help them float near
the surface of the ocean where they can get more sunlight. Assume that having many air bladders
(M) is a dominant trait. What gametes would be produced by a heterozygous individual?
½ M and ½ m
What gametes would be produced by a homozygous dominant individual?
All M
2. In humans, sickle cell anemia is a disease caused by a mutation of a single locus gene which
codes for an important blood protein. The allele for the normal protein (S) is dominant to that for
the one causing sickle cell anemia. What gametes would be produced by an individual who suffers
from sickle cell anemia?
All s (little s)
What gametes would be produced by individuals resulting from a SS x ss cross?
½ S and ½ s – the offspring of the cross SS x ss are all Ss
3. In areas of the very deep and damp southeastern United States lives what is, for all intents and
purposes, a giant flying cockroach known as a Palmetto bug. Assume that long antennae (L)
are dominant to short antennae and that normal wings (N) are dominant to bent ones.
What gametes will be produced by an individual that is homozygous dominant for antennae
and heterozygous for wings?
½ LN and ½ Ln
What gametes will be produced by an individual that is heterozygous for antennae
and homozygous recessive for wings?
½ Ln and ½ ln
4. In horses, the color black is due to a dominant allele (B) and chestnut color is due to
the recessive allele (b). The trotting gait is due to a dominant allele (T) and the pacing gait to
the recessive (t). What will be the gametes produced by a homozygous black pacer?
All Bt
Suppose we cross the black pacer above with a homozygous chestnut trotter. What will be
the gametes produced by the F1 generation?
¼ BT, ¼ Bt, ¼ bT and ¼ bt – the F1 generation would be heterozygous for both traits (BbTt)
5. Suppose you are designing a new Jurassic Park and you would like to have the crowd drawing
presence of Velociraptor without the danger of having your crowds eaten. You have managed to
come up with two varieties, very aggressive (A), which is apparently a dominant trait
in Velociraptors, and non-aggressive. They also come in two colors, red (R), which is dominant,
and green. At the moment you have a green female that is homozygous dominant for aggression
and a non-aggressive male that is homozygous dominant for color. What gametes will be produced
by the male and female?
Female:
all rA
Male:
all Ra
What gametes will be produced by the F1 generation of these two Velociraptors (providing of
course that the female doesn't dismember the male before mating)?
¼ RA, ¼ Ra, ¼ rA and ¼ ra – the F1 is result of crossing the two parents. They would all be
heterozygous for both traits (RrAa)
KEY: Monohybrid crosses
1. In dogs, wire hair (S) is dominant to smooth (s). In a cross of a homozygous wire-haired dog
with a smooth-haired dog, what will be the phenotype of the F1 generation?
All wire-haired
What would be the genotype?
All Ss
What would be the ratio of wire-haired to smooth-haired dogs in the F2 generation ?
3:1 (wire-haired : smooth)
2. Woodrats are medium sized rodents with lots of interesting
behaviors. You may know of them as packrats. Let's assume that the
trait of bringing home shiny objects (H) is controlled by a
single locus gene and is dominant to the trait of carrying home only
dull objects (h). Suppose two heterozygous individuals are crossed.
How many of each genotype would be expected if only 4 offspring
were produced?
HH: 1
Hh: 2
hh: 1
How many of each phenotype?
Brings home shiny objects: 3 - both HH and Hh genotypes lead to wire-haired phenotypes
Brings home dull objects:
1
3. Saguaro cacti are very tall cylindrical plants that usually have two L-shaped arms, one on each
side. Suppose you lived in southern Arizona where the Saguaro cactus is common and you happen
to have one growing in your yard. Your Saguaro has two arms but one is longer than the other.
Now, assume that arm length in these cacti are controlled by a single gene with arms of the same
length (A) being dominant to arms of different lengths. What is the genotype of your cactus?
aa
Could one of the parents of your cactus have had a phenotype with arms the same length?
yes
If so, what would have been the genotype of that parent?
Aa – while having the dominant phenotype, it must be able to pass on the recessive a allele
Suppose you cross your cactus with that of your neighbor which has arms of the same length. Your
great grandchildren (it takes a Saguaro cactus a long time to mature) find that half of the resulting
offspring have arms the same length and half have arms of different lengths. What was
the genotype of your neighbor's cactus?
Aa – if the other cactus had been AA, all of the offspring would have had arms of equal length
4. The common grackle is a species of robin-sized blackbirds that are fairly common (hence the
name) over most of the United States. Suppose that long tails (L) were dominant to short tails in
these birds. A female short-tailed grackle mates with a male long-tailed grackle who had one parent
with a long tail and one parent with a short tail. What is the male's genotype?
Ll – it must have inherited the recessive l from its short-tailed parent
How many of each genotype will be found in the F1 generation (assume 4 offspring)?
LL: 0
- since the female is ll and the male is Ll, there will be no LL offspring
Ll: 2
ll: 2
How many of each phenotype will be found in the F1 generation?
Long Tail:
2
Short Tail:
2 - see the genotypes above
KEY: Dihybrid crosses
1. About 70% of Americans perceive a bitter taste from the chemical phenylthiocarbamide (PTC).
The ability to taste this chemical results from a dominant allele (T) and not being able to taste PTC
is the result of having two recessive alleles (t). Albinism is also a single locus trait with normal
pigment being dominant (A) and the lack of pigment being recessive (a). A normally pigmented
woman who cannot taste PTC has a father who is an albino taster. She marries a homozygous,
normally pigmented man who is a taster but who has a mother that does not taste PTC. What are
the genotypes of the possible children (choose all that apply)?
TTAA: no
TtAA: yes
TTAa: no
TtAa: yes
TTaa: no
ttAA: yes
Ttaa: no
ttAa: yes
ttaa: no
- the parents are ttAa x TtAA
first parent produces tA and ta gametes
2nd parent produces TA and tA gametes
no offspring will have TT or aa; all other combinations are possible
What percentage of the children will be albinos?
0%
What percentage of the children will be non-tasters of PTC?
50%
2. Wolves are sometimes observed to have black coats and blue eyes.
Assume that these traits are controlled by single locus genes and are
located on different chromosomes. Assume further that normal coat
color (N) is dominant to black (n) and brown eyes (B) are dominant to
blue (b). Suppose the alpha male and alpha female of a pack (these are
the dominant individuals who do most of the breeding) are black with
blue eyes and normal colored with brown eyes, respectively. The female
is also heterozygous for both traits. How many of the offspring (assume
16) living in the pack will have each of the following genotypes?
NNBB:
NnBB:
NNBb:
NnBb:
NNbb:
Nnbb:
nnBB:
nnBb:
nnbb:
0
0
0
4
0
4
0
4
4
- the alpha male is nnbb. His gametes are all nb.
the alpha female is NnBb
her gametes are ¼ NB, ¼ Nb, ¼ nB and ¼ nb
none of their offspring will have NN or BB
the four possible genotypes will all be equally likely
What percent of the offspring will be normal colored with blue eyes?
25% - those with genotype Nnbb
3. In the breeding season, male Anole lizards court females by bobbing their heads up and down
while displaying a colorful throat patch. Assume for this question that both males and females bob
their heads and have throat patches. Assume also, that both traits are controlled by
single locus genes on separate chromosomes. Now, suppose that anoles prefer to mate with lizards
who bob their heads fast (F) and have red throat patches (R) and that these two alleles
are dominant to their counterparts, slow bobbing and yellow throats. A male
lizard heterozygous for head bobbing and homozygous dominant for the red throat patch mates
with a female that is also heterozygous for head bobbing but is homozygous recessive for yellow
throat patches. How many of the F1 offspring have the preferred fast bobbing / red
throat phenotype (assume 16 young)?
12 out 16 – all offspring will have red throat patches (they will be Rr); ¾ will have fast headbobbing (either FF or Ff)
What percentage of the offspring will lack mates because they have both slow head bobbing and
yellow throats?
0% - all will have the red throat patches
What percentage of the offspring will have trouble finding mates because because they lack one of
the dominant traits?
25% will have slow head-bobbing (ff genotype)
4. Carrion beetles lay their eggs in dead animals and then bury them in the ground until they hatch.
Assume that the preference for fresh meat (F) is dominant to the preference for rotted meat and that
the tendency to bury the meat shallow (S) is dominant to the tendency to bury the meat deep.
Suppose a female carrion beetle homozygous dominant for both traits mates with a male
homozygous recessive for both traits. What will be the genotype of the F1 generation?
All FfSs
What will be the phenotype of the F1 generation?
All Fresh and Shallow
What will be the genotypic ratio of the F2 generation (FFSS : FFSs : FFss : FfSS : FfSs : Ffss : ffSS
: ffSs : ffss)?
1 FFSS : 2 FFSs : 1 FFss : 2 FfSS : 4 FfSs : 2 Ffss : 1 ffSS : 2 ffSs : 1 ffss
5. Suppose in a strain of soybeans, high oil (H) content in the seeds is dominant to low oil content
and four seeds (E) in a pod is dominant to two seeds in a pod. A farmer crosses two soybean plants,
both with high oil content and four seeds per pod. The resulting F1 offspring have a phenotypic
ratio of 9:3:3:1 (High oil / four seeds : High oil / two seeds : Low oil / four seeds : Low oil / two
seeds). What genotype were the parent plants?
Both were HhEe
Suppose the farmer chooses two of the high oil / four seed plants and crosses them.
The F2 generation have all high oil / four seed phenotypes. What were the genotypes of the plants
chosen by the farmer to cross?
At least one of the plants is homozygous for each of the traits, but we cannot know for certain if
both are homozygous, or even whether either plant is homozygous for both traits.
Which known genotypes might the farmer cross her high oil / four seed plants with to determine
their genotype?
She should do a test cross – cross the individuals that have dominant phenotypes with one
that is recessive for both: hhee
If the dominant individual is heterozygous for either trait (or both), it will be clear when some
of the offspring have the recessive phenotype.
KEY: Incomplete dominance
1. In northeast Kansas there is a creature know as a wildcat. It comes in three
colors, blue, red, and purple. This trait is controlled by a single locus gene
with incomplete dominance. A homozygous (BB) individual is blue, a
homozygous (bb) individual is red, and a heterozygous (Bb) individual is purple. What would be
the genotypes and phenotypes of the offspring if a blue wildcat were crossed with a red one?
Genotype: all Bb
Phenotype: all purple
What are the genotypic and phenotypic ratios of the F2generation?
Genotypic ratio (BB:Bb:bb): 1 BB: 2 Bb : 1 bb
Phenotypic ratio (Blue:Purple:Red): 1 blue : 2 purple : 1 red
2. The lubber grasshopper is a very large grasshopper, and is black with red and yellow stripes.
Assume that red stripes are expressed from the homozygous RR genotype, yellow stripes from the
homozygous rr genotype, and both from the heterozygous genotype. What will be the phenotypic
ratio of the F1 generation resulting from a cross of two grasshoppers, both with red and yellow
stripes (red : both : yellow)? Technically, this is an example of codominance!
1 red : 2 both : 1 yellow
What would be the genotypic ratio of the F1 generation (RR : Rr : rr)?
1 RR ; 2 Rr : 1 rr
What genotypes would be produced by crossing a grasshopper with both color stripes and one with
yellow stripes (choose all that apply)?
Rr and rr (with a 1 : 1 ratio)
What phenotypes would be produced by crossing a grasshopper with both color stripes and one
with yellow stripes (choose all that apply)?
both red & yellow and just yellow (with a 1 : 1 ratio)
3. Suppose you have two rose plants, both with pink flowers. You cross the two plants and are
surprised to find that, while most of the offspring are pink, some are red and some are white. You
decide that you like the red flowers and would like to make more. What cross would you perform
to produce the most red flowered plants?
Cross two red-flowered plants (they are both homozygous for red)
Your mother decides she would like some of the pink flowered roses. Which cross would give you
the most pink flowered plants?
Cross a red (RR) and a white-flowered (rr) plant – all of their offspring will be pink (Rr)
4. A naturalist visiting an island in the middle of a large lake observes a species of small bird with
three distinct types of beaks. Those with short, crushing beaks (BB) consume hard shelled nuts,
those with long, delicate beaks (bb) pick the seeds from pine cones, and those with intermediate
beaks (Bb), consume both types of seeds though they are not as good at either. Assume that this
difference in beak morphology is the result of incomplete dominance in a single locus gene. Which
of the mated pairs below will have the best adapted offspring in a year in which most of the food
available is in the form of hard shelled nuts?
The offspring of two birds with short, crushing beaks (BB) will all have short, crushing beaks
What would be the phenotypic ratio of the F1 generation resulting from a cross of Bb x bb
(Short:Intermediate:Long)?
1 intermediate : 1 long
How many offspring of an intermediate x short beak cross will have long beaks (assume 4)?
2 should have long beaks (bb) – the cross produces ½ Bb and ½ bb
5. Racoons have rings around their tails and a habit of washing their food in water before eating it.
Suppose that both of these traits are controlled via incomplete dominance so that wide bands on the
tail are BB, medium sized bands are Bb, and narrow bands are bb and that washing all their food is
WW, washing some of their food is Ww, and washing no food is ww. How many of each genotype
will be in the F1 generation resulting from a cross of two racoons, both with medium sized tail
bands and that wash some of their food (assume 16)?
both parents are BbWw
BBWW: 1
BBWw: 2
BBww: 1
BbWW: 2
BbWw: 4
Bbww:
2 - note that this genotype was missing on the original problem set
bbWW: 1
bbWw:
2
bbww:
1
How many of the F1 generation will have wide tail bands and won't wash any of their food?
Only 1 – with genotype BBww
KEY: Sex-linked genes
1. A boy, whose parents and grandparents had normal vision, is color-blind. What are
the genotypes for his mother and his maternal grandparents. Use XB for the dominant normal
condition and Xb for the recessive, color-blind phenotype.
Mother:
since all have normal color vision, both mother and grandmother are carriers
XBXB
XBXb
XbXb
Grandmother:
XBXB
XBXb
XbXb
Grandfather:
XBY
XbY
2. The bison herd on Konza Prairie has begun to show a
genetic defect. Some of the males have a condition known
as "rabbit hock" in which the knee of the back leg is
malformed slightly. We do not yet know
the genes controlling this trait but for the sake of our
question, we shall assume it is a sex-linked gene and that it
is recessive. Now, suppose that the herd bull (the dominant
one which does most of the breeding) who is normal (XN)
mates with a cow that is a carrier for rabbit hock. What are his chances of producing a normal son?
½ of his male offspring will be normal (the bull passes on a Y; the cow is XNXn)
If he mates with this cow every year, what percentage of their daughters have normal knees?
All of their female offspring will have normal knees, because they all inherit the bull’s XN
What percentage of their daughters will be carriers of rabbit hock?
50% of the female offspring will be carriers, because they inherit the female’s Xn
3. A woman with red-green color-blindness has a mother with normal vision. Knowing that colorblindness is a sex-linked recessive gene, can you determine what her father's phenotype is?
yes
If so, what is it?
the father is colorblind (the woman must be XrXr, so her mother is a carrier and her father is XrY)
The woman marries a man with normal vision. What is the probability they will have sons who are
red-green color-blind?
All of their sons will be color-blind (they all get a Y from their dad and an Xr from their mom)
What is the probability they will have daughters who are red-green color-blind?
None of their daughters will be colorblind because they all inherit their dad’s XR chromosome
4. A rancher owns a bull with many desirable characteristics. Unfortunately, he also has a sexlinked trait that in the recessive form leads to no pigment formation in the iris of the eye. This
makes the bull very sensitive to sunlight and could lead to blindness. The rancher wishes to breed
him to a cow that will minimize the chances of any offspring showing this trait. She would
especially like to produce another bull with most of his sire's desirable qualities but without the
non-pigmented eye. Two cows with the dominant normal colored eyes (XN) are available that have
been genetically typed for this particular trait. Cow 1 has a genotype of XN XN and cow 2 is XNXn.
Which of these two cows should the rancher choose as a mate to her bull if she wishes to minimize
the occurrence of the non-pigmented eye in his offspring?
she should choose the homozygous one (XNXN)
What percentage of the male offspring from the preferred cross will have non-pigmented eyes?
0% - all the males inherit the bull’s Y chromosome and the female’s XN
Will crossing the bull with this cow eliminate the trait from the herd? No
If not, why not?
The dominant alleles of the female will override the recessive from the male.
There are more males than females produced, increasing the presence of the Xn allele.
All female offspring of this cross are heterozygous, so they carry the Xn allele and can pass it on to their
offspring.
Male offspring will automatically have the Xn allele because their father did.
5. Clouded leopards are a medium sized, endangered species of
cat, living in the very wet cloud forests of Central America.
Assume that the normal spots (XN, pictured here) are a dominant,
sex-linked trait and that dark spots are the recessive counterpart.
Suppose as a Conservation Biologist, you are involved in a
clouded leopard breeding program. One year you cross a male
with dark spots and a female with normal spots. She has four cubs and, conveniently, two are male
and two female. One each of the male and female cubs have normal spots and one each have dark
spots. What is the genotype of the mother?
XNXN
XNXn
XnXn
the male passes on his Xn to all female offspring; the mother passes Xn to ½ of her offspring
Suppose a few years later, you cross the female cub that has normal spots with a male that also has
normal spots. How many of each genotype will be found in the cubs (assume 4)?
XNXN:
XNXn:
XnXn:
XNY:
XnY:
- the female is XNXn; the male is XNY
1
1
0
1
1
Will any of the cubs from this latest cross have dark spots?
yes
If so, how many and of what sex will they be?
Male:
1
Female:
0